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W.A.V.S. Compression

W.A.V.S. Compression. Alex Chen Nader Shehad Aamir Virani Erik Welsh. Overview. Approach Psychoacoustic Modeling Filter Banks Quantization Demonstration Results Further Research. Approach. Encoding:. Filter Banks. Quantization. Input. Encoded Signal. Psychoacoustic Model.

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W.A.V.S. Compression

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  1. W.A.V.S. Compression Alex Chen Nader Shehad Aamir Virani Erik Welsh

  2. Overview • Approach • Psychoacoustic Modeling • Filter Banks • Quantization • Demonstration • Results • Further Research

  3. Approach Encoding: Filter Banks Quantization Input Encoded Signal Psychoacoustic Model Decoding: Encoded Signal Inverse Quantization Reconstruction Filter Banks Output

  4. Psychoacoustic Model • Based on studies that show hearing capabilities affected by: • Environment • Limitations of human auditory system • Used to eliminate portions of signal average human won’t hear • Two key properties: • Absolute threshold of hearing • Auditory masking

  5. Absolute Threshold of Hearing • Experiment: • Plot audible threshold of tone • Observations: • Auditory system sensitive to some frequencies • Frequencies within “critical bandwidth” treated similarly • Basis for Bark scale

  6. Auditory Masking • Tones and noise drown out less powerful sounds • Affect neighboring frequencies • Affect critical bandwidth • Effects add to produce overall masking threshold • Mask quantization

  7. Filter Banks Theory • Array of bandpass filters • Break up signal into frequency subbands • Allows for variable coding scheme

  8. Analysis and Synthesis Banks 1) Analysis filters divide up the signal 2) Down-sample 3) Quantize • 4) Up-sample • 5) Synthesis filters remove distortions • 6) Reconstruct the signal

  9. Filter Bank Design • Phase • Tradeoff between fine and coarse frequency resolution • Piccolo vs. Castanets • Non-stationary signals • We used non-adaptive approach

  10. Filter Bank Implementation • We used Cosine Modulated PR (perfect reconstruction) filter banks with 32 filters each • Output is a delayed version of the input (linear phase) • Distortion arises from quantization only

  11. Quantization Two types • Narrow-band • Current input • Overhead cost • Full-range • Independent of current input • No overhead Sampled Input Quantized Version Reconstructed Input

  12. Quantization • Narrow Band • More accurate • Lower compression ratio • Full-Range • Less accurate • Higher compression ratio • Using 3-bit Quantization • Input: -.4 -.22 .14 .4 • Levels: 1 3 6 8 • Recon.: -.4 -.2 .1 .3 • Total Error: .16 • Input: -.4 -.22 .14 .4 • Output: 3 4 6 7 • Recon: -.5 -.25 .25 .50 • Total Error: .34

  13. Sine wave Full range Narrow range Chime 8-bit Full range Narrow range Percussion Full Range Narrow Range Modern 8-bit Full Range Narrow Range Demonstration

  14. Sine Wave (time) Full-Range Quantization Narrow Quantization

  15. Sine Wave (freq) Full-Range Quantization Narrow Quantization

  16. Sine Wave (freq error) Full-Range Quantization Narrow Quantization

  17. Modern (time) Full-Range Quantization Narrow Quantization

  18. Modern (freq) Full-Range Quantization Narrow Quantization

  19. Modern (freq error) Full-Range Quantization Narrow Quantization

  20. Results • Full Range: Smallest File, Worst Sound Quality • Narrow Range: Better Sound Quality, Larger File • MP3: Industry Standard

  21. Further Research • Filter Banks • Wavelets • Dynamic Frequency Ranges • Better Psychoacoustic Model • Tone Designation • Pre- and Post- Echo • Bit Allocation • Writing a File

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